A novel and facile decay path of Criegee intermediates by intramolecular insertion reactions via roaming transition states Trong-Nghia Nguyen, Raghunath Putikam, and M. C. Lin Citation: The Journal of Chemical Physics 142, 124312 (2015); doi: 10.1063/1.4914987 View online: http://dx.doi.org/10.1063/1.4914987 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/142/12?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Theoretical prediction of rare gas inserted hydronium ions: HRgOH2 + J. Chem. Phys. 138, 194308 (2013); 10.1063/1.4804623 Vibrationally quantum-state-specific dynamics of the reactions of CN radicals with organic molecules in solution J. Chem. Phys. 134, 244503 (2011); 10.1063/1.3603966 Trends in C–O and C–N bond formations over transition metal surfaces: An insight into kinetic sensitivity in catalytic reactions J. Chem. Phys. 126, 194706 (2007); 10.1063/1.2734544 Transition state dynamics of N 2 O 4 ⇌2 NO 2 in liquid state J. Chem. Phys. 120, 10127 (2004); 10.1063/1.1735596 Addition-insertion-elimination reactions of O ( 3 P) with halogenated iodoalkanes producing HF (v) and HCl (v) J. Chem. Phys. 114, 2251 (2001); 10.1063/1.1337800
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THE JOURNAL OF CHEMICAL PHYSICS 142, 124312 (2015)
A novel and facile decay path of Criegee intermediates by intramolecular insertion reactions via roaming transition states Trong-Nghia Nguyen,1,2 Raghunath Putikam,1 and M. C. Lin1,a) 1
Department of Applied Chemistry and Institute of Molecular Science, National Chiao Tung University, Hsinchu 30010, Taiwan 2 Department of Physical Chemistry, Hanoi University of Science and Technology, Hanoi, Vietnam
(Received 2 December 2014; accepted 3 March 2015; published online 27 March 2015) We have discovered a new and highly competitive product channel in the unimolecular decay process for small Criegee intermediates, CH2OO and anti/syn-CH3C(H)OO, occurring by intramolecular insertion reactions via a roaming-like transition state (TS) based on quantum-chemical calculations. Our results show that in the decomposition of CH2OO and anti-CH3C(H)OO, the predominant paths directly produce cis-HC(O)OH and syn-CH3C(O)OH acids with >110 kcal/mol exothermicities via loose roaming-like insertion TSs involving the terminal O atom and the neighboring C–H bonds. For syn-CH3C(H)OO, the major decomposition channel occurs by abstraction of a H atom from the CH3 group by the terminal O atom producing CH2C(H)O–OH. At 298 K, the intramolecular insertion process in CH2OO was found to be 600 times faster than the commonly assumed ring-closing reaction. C 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4914987]
I. INTRODUCTION
Criegee intermediates (CIs) are key reaction intermediates in the reactions of ozone (O3) with olefins1 and the reaction of O2 with 3CH2, a key intermediate in the combustion of hydrocarbons.2 Small CIs have been recently detected experimentally,3–7 including the 2 conformers of acetaldehyde oxide, syn-CH3C(H)OO and anti-CH3C(H)OO, which differ in the orientations of the C–O–O group.4(b) Of the CIs formed during ozonolysis, ∼50% of them decompose to produce OH on a sub second timescale while the other 50% are stabilized.8–10 These CIs can then react with other components in the air or decompose over a much longer lifetime, t < 0.3 s, forming OH radicals. Bimolecular reactions between CIs and H2O, SO2, NO2, CH3C(O)H, CF3C(O)CF3, CH3C(O)CH3, HCHO, and so on have been reported in the literature.8,9,11–18 Experimental studies on the CH2OO reactions with SO2 and NO2 proved to be unexpectedly rapid, implying a substantially greater role of the carbonyl oxides in models of tropospheric sulfate and nitrate chemistry than previously assumed.4(a) The reactions of the small CIs, CH2OO and CH3CHOO, with carbonyl species were measured by laser photolysis/tunable synchrotron photoionization mass spectrometry.11 The results showed that the rate coefficient for CH2OO + hexafluoroacetone was k = (3.0 ± 0.3) × 10−11 cm3 molecule−1 s−1, supporting the possible use of hexafluoroacetone as a Criegee-intermediate scavenger. The mechanism for decomposition of CIs has been reported in the literature.1(c),17–19 Sander found evidence that the O(3P) atom was ejected in the reaction of a phenyl carbene with O2 by RR′COO → RR′CO + O(3P).18(b) However, for the singlet H2COO(1A′′), Anglada et al.18(c) using complete a)Author to whom correspondence should be addressed. Electronic mail:
[email protected]. 0021-9606/2015/142(12)/124312/6/$30.00
active space self-consistent field (CASSCF) and multireference single and double configuration interaction (MRDCI) calculations showed that it correlates with the singlet products H2CO(1A1) and O(1D), with a much higher energy than H2COO(1A′′). Therefore, the lower possible reaction paths of the singlet H2COO(1A′′) have been suggested,18(a),18(c)
For anti-CH3C(H)OO, a similar decomposition mechanism is generally believed to give OH + CH3CO, CH4 + CO2, CH2CO + H2O, and CH3OH + CO.1(c) For syn-monosubstituted CIs, such as syn-CH3C(H)OO, tautomerization to the corresponding hydroperoxide involving a 1,4 H-atom migration from the methyl to the terminal O appears to be a competitive isomerization pathway.1(c),18(c) However, in the above calculations for the small Criegee systems, uncertainties still exist regarding the tight transition states (TSs) forming dioxiranes from CH2OO and antiCH3CHOO. Recent multireference calculations show that Criegee intermediate, CH2OO, is predominantly zwitterionic with only weak biradical character.5,20 In addition, under a high concentration condition, the zwitterionic CH2OO was recently found to decay bimolecularly via a 6-member-ring dimeric intermediate by a head-to-tail association mechanism rapidly producing exothermic products, 2 CH2O + O2(1∆) (∆rH = −75.9 kcal/mol).5(b) Under a low concentration condition, in addition to the dioxirane cyclization path, is it also possible for the very exothermic intramolecular reaction directly transforming CH2OO to HCOOH? Such a unimolecular decay process is not unlikely on grounds that the terminal O atom should at least partially possess the O(1 D) character
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when the O–O bond is stretched; if so, it may undergo insertion into a cis-C–H bond forming HC(O)OH and CH3C(O)OH with >110 kcal/mole exothermicities which may enthalpically drive the reactions into the deep wells of the acids. HCOOH and CH2OO are known products of the Criegee reaction involving O3 and C2H4. There is no experimental measurement for the formation of formic acid from the unimolecular reaction CH2OO under clean molecular beam conditions however. Welz et al.4(a) have reported the direct photoionization mass spectrometric detection of formaldehyde oxide (CH2OO) as a product of the reaction of CH2I with O2. Their photoionization spectrum of the m/z = 46 species agrees with that experimentally measured system of CH2OO in chlorine-initiated dimethyl sulfoxide (DMSO) oxidation3 and with coupledcluster with single and double and perturbative triple excitations/complete basis set (CBS) extrapolation calculations for the CH2OO ionization energy.20(a) Dioxirane and formic acid have much higher ionization energies, 10.82 eV and 11.33 eV,16,17 respectively. The mass and the photoionization spectra have well supported the product as formaldehyde oxide, CH2OO. At longer times, a small formic acid signal was observed which may be produced by the isomerization of CH2OO. In this work, we have carried out a detailed mapping of the potential energy surfaces (PESs) of the two small CIs with a particular emphasis on the search for the existence of the intramolecular insertion mechanism via a loose transition state in which the terminal O atom exhibits its O(1D) character. Such a novel reaction path was indeed found for the first time via a roaming-like transition state21 and shown to be a facile decay path in both systems as well. II. COMPUTATIONAL METHODS
We have characterized the mechanisms of these two reactions by quantum-chemical calculations using hybrid density functional theory (B3LYP) and MP2 methods with the aug-ccpVTZ basis set to optimize geometries of all species involved by means of the Gaussian 09 package.22 Further improvement of energetics of the PESs has been made by singlepoint calculations with CCSD(T)/aug-cc-pVTZ. The predicted full PESs of CH2OO and CH3CHOO are presented in Figures S1 and S2 of the supplementary material23 and the corresponding lower energy paths are shown in Figures 1 and 2, respectively. The transition state geometries are used as an input for intrinsic reaction coordinate (IRC) calculations to verify the connectivity of the reactants and products for further confirmation. For the new roaming-like loose transition state of the CH2OO system, it has been confirmed with optimizations using several different methods including UB3LYP, UBLYP, UCCSD, and CASSCF(14,12) employing the aug-cc-pVTZ basis set. To more accurately evaluate the energies, we employed the UCCSD(T)/CBS method24 based on the CCSD/augcc-pVTZ optimized geometries for the two critical transition states, TS1 and TS2, of CH2OO to be described in detail below. The CBS energies were evaluated with these geometries as follows. The total energies E(X) computed with the aug-ccpVXZ basis sets (X = 2, 3, 4) extrapolated to the CBS limits ECBS employing a three-point extrapolation scheme,24 E (X) = ECBS + b exp [− (X − 1)] + c exp[−(X − 1)2],
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FIG. 1. Energy diagrams for the CH2OO decomposition. All values (in kcal/mol at 0 K) were computed at the CCSD(T)/aug-cc-pVTZ//B3LYP/augcc-pVTZ level. The values in parentheses are calculated by using the UCCSD(T)/CBS//UCCSD/aug-cc-pVTZ level. Data in the square bracket are predicted at CCSD(T)/aug-cc-pVTZ//CAS(8,8)/cc-pVTZ level.
where E(X) is the single point energy calculated by UCCSD (T)/aug-cc-pVTXZ method, X is the cardinal number of the basis sets connected with X = 2 (DZ), 3 (TZ), 4 (QZ), and ECBS, b, and c are parameters to be fitted. Calculations of the micro canonical rate coefficients for the low-lying reaction channels were made with the Chemrate program25 based on the Rice–Ramsperger–Kassel–Marcus (RRKM) theory with Eckart tunneling corrections. Chemical activation, isomerization, and decomposition were accounted for with
FIG. 2. The IRC profile for the loose TS1, computed at the UB3LYP/aug-ccpVTZ level.
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the program through the solution of the time-dependent onedimensional master equation. In the calculation of the specific rate constant, the number of available states in the transition state is obtained at the energy E and with the total angular momentum J resolved based on the rigid-rotor harmonic oscillator (RRHO) assumption for the energy levels. Our predicted rate constants were calculated from conventional (fixed geometry) canonical transition-state theory. More detailed information is available in the supplementary material.23
III. RESULTS AND DISCUSSION A. Potential energy surface of CH2OO decomposition
Aside from the commonly invoked ring cyclization reaction producing cyc-H2COO (IS3), we have investigated three other possibilities: H-atom migration to the neighboring and terminal O atoms producing HCO(H)O and HCO–OH, respectively, as well as the intramolecular O-atom insertion into a C–H bond as alluded to above, when the terminal O-atom is stretched along the O–O dissociation path producing CH2O + O(1D), whose dissociation limit is now known to require a rather high energy (∼54 kcal/mol)26 and thus is not accessible thermochemically under ambient atmospheric conditions. The optimized geometries of CH2OO bond lengths are in good agreement with the experimental and theoretical data as shown in Table S1.5,7,23 The vibrational frequencies of all the species involved in these reactions are predicted at the B3LYP/aug-cc-pVTZ level and are given in Table S2.23 The new roaming-like mechanism will be discussed below after the presentation for the ring cyclization and H-transfer processes. The formation of the cyclic cyc-H2COO (IS3: −25.7 kcal/mol) occurs via transition state TS2 (18.7 kcal/mol); the IRC result shows that TS2 connects IS3 with CH2OO. These results are in good agreement with those in the literature1(c),27(a),27(b) (Table S3 and Figure S323). The relative energies of IS3 and TS2 predicted by CCSD(T)/aug-ccpVTZ//B3LYP/aug-cc-pVTZ, −25.7 and 18.7 kcal/mol, and by CCSD(T)/aug-cc-pVTZ//MP2/aug-cc-pVTZ, −26.5 and 18.7 kcal/mol, are close to −25.1 and 19.6 kcal/mol, respectively, by Fang et al.27(a) at the CAS(8,6)+1+2/cc-pVDZ level of theory (Table S323). Because of the low barrier, 18.7 kcal/mol, this channel has been commonly assumed to be predominant and rate-controlling in the fragmentation of CH2OO. The next two channels involve H-migration forming HCO–OH (IS8: 7.1 kcal/mol) via TS16 lying 30.4 kcal/mol above the reactant and HCO(H)O (IS9: 70.5 kcal/mol) via TS17 lying 92.6 kcal/mol above the reactant. Clearly, these processes are insignificant because of their high energy barriers and endothermicities (Figure S1).23 On the other hand, the direct transformation of CH2OO into c–HC(O)OH (IS1) is very exothermic (−111.3 kcal/mol); as alluded to above, such a transformation process was indeed found after an exhaustive manual search. The very loose transition state TS1 was found to lie 16.1 kcal/mol above CH2OO or 2.6 kcal/mol below the commonly known TS2 (18.7 kcal/mol) for cyclization; the barrier difference is similar to the UCCSD(T)/CBS//UCCSD/aug-cc-pVTZ
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value, 2.2 kcal/mol (Figure 1). TS1 has a unique imaginary frequency of 273.7i corresponding to the insertion of the terminal O atom into a C–H bond. The H–O and O–O bond distances, 1.997 and 2.639 Å, are close to the structure of the transition state corresponding to those for the insertion of the O atom.28 The IRC profile of TS1 is displayed in Figure 2. The result clearly shows that TS1 does in fact smoothly connect CH2OO and c-HC(O)OH. The existence of this roaming-like TS, as mentioned above, has been confirmed with optimizations using several computational methods: UB3LYP, UBLYP, UCCSD, and CASSCF(14,12) employing the aug-cc-pVTZ basis set. The optimized geometries with all the methods are shown in Figure S4.23 The corresponding barrier energies and vibrational frequencies estimated at different levels of the theory are summarized in Tables S4 and S5,23 respectively. As revealed by the PES, there are two possible reaction paths for cycH2COO (dioxymethane) decomposition: H2 elimination and ring opening reactions. First, the intermediate dioxymethane can dissociate by H2 elimination through TS6 to yield H2 + CO2 with a barrier of 24.9 kcal/mol. In the second mechanism by ring opening via TS6a, we were unable to locate the TS and open-H2C(O)O with the B3LYP method; it was, however, located with a higher level of theory by CCSD(T)/aug-cc-pVTZ//CAS(8,8)/cc-pVTZ. As shown in the PES, cyc-H2COO can decompose into open-H2C(O)O by O–O bond breaking via TS6a with a 21.1 kcal/mol barrier, to be followed by a rapid H-atom migration to the neighboring O atom to produce the trans-formic acid (t-HC(O)OH) via TS6b with a small 2.4 kcal/mol barrier. The optimized geometries of these processes are shown in Figure S5.23 Anglada and coworkers19(c),27(b) have studied the detailed decomposition mechanism of cyc-H2COO at the CASPT2/6-31g(d,p)//CASSCF/6-31g(d,p) level of theory. Their barriers for the ring opening and H-migration were reported to be 18.5 and 1.9 kcal/mol, respectively.27(b) Fang et al.27(a) also have calculated the similar barrier energies at the CAS(8,6)+1+2/cc-pVDZ level to be 19.9 and 1.6 kcal/mol. These results are consistent with our values cited above. For CH2OO decomposition, TS1 and TS2 are the rate-controlling steps; the details of other remaining dissociation processes are shown in Figures 1 and S1.23 The thermal decomposition of 1CH2OO producing 1H2CO + O(1D) and 3 H2CO + O(3P) products predicted at the CCSD(T)/aug-ccpVTZ//B3LYP/aug-cc-pVTZ level are endothermic by 54.2 and 74.2 kcal/mol, respectively. Clearly, the 3H2CO + O(3P) formation is not favored energetically. In addition, we have constructed the approximate minimum energy path (MEP) for 3 H2CO + O(3P) formation by freezing the 3H2CO structure when it interacts with the O(3P) atom as shown in Fig. S6,23 computed at the CASPT2/aug-cc-pVTZ//CAS(8,8) augcc-pVTZ level. Significantly, at any point on the triplet product curve, a full optimization immediately converges the structure and energy onto the singlet MEP curve. As shown in Fig. S6,23 a full optimization at 3.0 Å, when O atom approaches near the bend 3CH2O, it immediately converges to the 1CH2O–O(1D) curve with the release of over 70 kcal/mol π bond energy. Thus, the unimolecular decomposition of the singlet CH2OO is not favored to produce the 3H2CO + O(3P)
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products. In addition, detailed kinetics and mechanism for the decomposition of HCOOH have been reported previously by Chang et al.27(c) Based on the mechanism, the highly excited c-HC(O)OH thus formed can decompose barrierlessly giving HCO + OH (P3: −9.9 kcal/mol), H2 + CO2 (P1: −119.6 kcal/mol) via TS5 (−45.6 kcal/mol), or t-HC(O)OH (IS5: −115.3 kcal/mol) via a low isomerization barrier TS3 (−104.0 kcal/mol). Similarly, t-HC(O)OH can decompose barrierlessly forming HCO + OH (P3: −9.9 kcal/mol) or H2O + CO (P2: −111.3 kcal/mol) via TS4 (−48.8 kcal/mol). Our predicted energies are in good agreement with previous studies27 (Table S323). The direct CH2OO → HC(O)OH channel via the novel roaming-like TS1 not only has a very loose transition structure but also has the lowest energy barrier, 16.1 kcal/mol. The main products of the reaction are expected to be OH, H, CO, and CO2 from the fragmentation of the highly excited HC(O)OH as observed experimentally.1(c),9,16,27,29 B. Potential energy surface of CH3C(H)OO decomposition
The detailed PES of the CH3CHOO system is presented in Figure S2 in the supplementary material,23 only the abbreviated one depicting the lowest energy paths is shown in Figure 3. The related geometries are shown in Figure S7.23 Corresponding vibrational frequencies of all species calculations at the B3LYP/aug-cc-pVTZ level are summarized in Table S6.23 There are two conformers of CH3C(H)OO, the anti-structure (a-CH3C(H)OO) and the synstructure (s-CH3C(H)OO), with the former having a higher energy, 3.5 kcal/mol. The predicted energy barrier for the isomerization, anti → syn, is as high as 38.1 kcal/mol. Therefore, it should have different reactivities for these conformers. These results are in good agreement with
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experimental studies,4(b),10,11 in which the two conformers were independently detected, and the fit parameters suggested a far larger overall production (90%) of the more stable synconformer, assuming that the electronic transition moments for the two conformers were similar. Our result reveals that the decomposition of aCH3C(H)OO has two lowest energy channels, 15.4 and 15.6 kcal/mol, respectively, to the cyclization giving cycCH3C(H)OO (IS6: −26.3 kcal/mol) via TS8 and the intramolecular insertion forming s-CH3C(O)OH (IS4: −115.3 kcal/mol) via TS7, which has a unique imaginary frequency of 126.3i with the H–O and O–O bond distances, 1.956 and 2.573 Å. The IRC analysis also confirms the transition state TS7 (Figure S8).23 The predicted relative energies of TS8 and the cyc-CH3C(H)OO at the CCSD(T)/augcc-pVTZ//B3LYP/aug-cc-pVTZ level, 15.4 kcal/mol and −26.3 kcal/mol, agree closely with the values, 15.4 kcal/mol and −23.44 kcal/mol, 17.55 kcal/mol and −28.2 kcal/mol, using MCG3//QCISD/MG3 and BB1K/6-31+G(d,p), respectively.30 Although the two channels have similar energy barriers, the intramolecular insertion via TS7 is expected to be dominant because of its very loose roaming-like TSstructure as illustrated below with the predicted rate constants. The major products of the reaction should, therefore, derive primarily from the decomposition of a/s-CH3C(O)OH, such as OH, H, and CO2, consistent with experimental results.1(c),9,10,16,20,27–30 The decomposition of the s-CH3C(H)OO has the lowest energy channel, 16.6 kcal/mol, corresponding to the abstraction of a H atom in CH3 by the terminal O atom forming CH2C(H)O–OH (IS7: −22.2 kcal/mol) via TS12. Our barrier energy is in good agreement with the experimental value of 16.0 kcal/mol reported by Liu et al.10(b) Recent theoretical results give ∼17.9 kcal/mol.10(c),30 However, the attacking of the terminal O atom at the C atom forming cyc-CH3C(H)OO (IS6) via TS13 has to overcome 23.3 kcal/mol barrier. These results are in close agreement with previous theoretical studies (Table S7).23 C. Rate constant calculations
We have predicted the microcanonical rate coefficients and the thermal rate constants for the unimolecular reactions of CH2OO and anti/syn-CH3C(H)OO producing various products via insertion and ring formation paths based on the TS’s presented above computed at the CCSD(T)/aug-ccpVTZ//B3LYP/aug-cc-pVTZ level as follows: CH2OO → c-HC (O) OH (k 1) , CH2OO → cyc-H2C (O) O (k2) , a-CH3C (H) OO → CH3C (O) OH (IS4) (k3) , a-CH3C (H) OO → c-CH3C (H) OO (k 4) , s-CH3C (H) OO → CH2C (H) OO(IS7) (k5) , s-CH3C (H) OO → c-CH3C (H) OO(IS6) (k6) . FIG. 3. Energy diagrams for the anti/syn-CH3C(H)OO decomposition. All values (in kcal/mol at 0 K) were computed at the CCSD(T)/aug-ccpVTZ//B3LYP/aug-cc-pVTZ level.
The vibrational frequencies of all the species involved in these reactions are calculated at the B3LYP/aug-cc-pVTZ level and are listed in Tables S2 and S6 for the kinetic
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298 K values.23 The microcanonical rate coefficients for both CH2OO and CH3C(H)OO decomposition channels are shown in Figure 4 to illustrate their relative importance. In Figure 4(a), k 1(E) for generation of c-HC(O)OH via the insertion process is greater than k2(E) for the formation of cyc-H2C(O)O via ring formation by 2-3 orders of magnitude over a wide range of energies and the integrated thermal rate constant at 298 K for the former is 600 times greater. The rate coefficients for the anti-CH3C(H)OO decomposition as shown in Figure 4(b), k3 (E) for CH3C(O)OH(IS4) via insertion into the CH group is also dominant over that for the ring formation producing cyc-CH3C(H)OO. In the case of syn-CH3C(H)OO shown in Figure 4(c), H migration giving CH2C(H)OOH (IS7) (k5) is dominant. It should be mentioned that our effort to look for the existence of a transition state for insertion of the terminal O atom into a C–H bond of the methyl group failed to locate it after an extensive search.
IV. CONCLUSION
To summarize, the PESs and decomposition mechanisms of CH2OO and syn/anti-CH3C(H)OO have been computed at the CCSD(T)/aug-cc-pVTZ//B3LYP/aug-cc-pVTZ and CCSD(T)/aug-cc-pVTZ//MP2/Aug-cc-pVTZ levels; the energy dependent decomposition rate constants have been predicted. The results show that the isomerization of CH2OO forming cis-HC(O)OH via a very loose roaming-like insertion TS lying 16.1 kcal/mol above CH2OO is very facile and could be competitive with the fastest known channels. CH3C(H)OO has two structures, anti- and syn-, in which the syn-structure is more stable, and the inter-conversion (anti → syn) energy barrier is rather high, 38.1 kcal/mol. The major channel in the decomposition of anti-CH3C(H)OO is the insertion of the terminal O atom into the neighboring C–H bond forming syn-CH3C(O)OH, whereas the dominant channel in the syn-CH3C(H)OO decomposition reaction is the abstraction of a H atom from the CH3 group by the terminal O atom producing CH2C(H)O–OH. In both systems, the commonly assumed cyclization process producing dioxiranes with their subsequent isomerization/decomposition reactions is much less competitive. ACKNOWLEDGMENTS
FIG. 4. Predicted rate coefficients of the unimolecular decomposition channels of (a) CH2OO, (b) anti-CH3CHOO, and (c) syn-CH3CHOO, as a function of energy calculated by using CCSD(T)/aug-cc-pVTZ//B3LYP/aug-cc-pVTZ method.
calculations.23 The high-pressure and low-pressure limit rate constants for the dissociation processes predicted for the 200-1000 K temperature range obtained by least-squares fitting have been presented in Table S8 along with the
The Ministry of Science and Technology of Taiwan (Grant No. NSC102-2745-M009-001-ASP) and the Ministry of Education, Taiwan (“Aim for the Top University Plan” of National Chiao Tung University) supported this work. The National Center for High-performance Computing provided computer time. 1(a)
R. Criegee and G. Wenner, Justus Liebigs Ann. Chem. 564, 9 (1949); (b) R. Criegee, Angew. Chem., Int. Ed. Engl. 14, 745 (1975); (c) D. Johnson and G. Marston, Chem. Soc. Rev. 37, 699 (2008). 2W. R. Wadt and W. A. Goddard, J. Am. Chem. Soc. 97, 3004 (1975). 3C. A. Taatjes, G. Meloni, T. M. Selby, A. J. Trevitt, D. L. Osborn, C. J. Percival, and D. E. Shallcross, J. Am. Chem. Soc. 130, 11883 (2008). 4(a) O. Welz, J. D. Savee, D. L. Osborn, S. S. Vasu, C. J. Percival, D. E. Shallcross, and C. A. Taatjes, Science 335, 204 (2012); (b) C. A. Taatjes, O. Welz, A. J. Eskola, J. D. Savee, A. M. Scheer, D. E. Shallcross, B. Rotavera,
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E. P. F. Lee, J. M. Dyke, D. K. W. Mok, D. L. Osborn, and C. J. Percival, Science 340, 177 (2013). 5(a) Y. T. Su, Y. H. Huang, H. A. Witek, and Y. P. Lee, Science 340, 174 (2013); (b) Y. T. Su, H. Y. Lin, P. Raghunath, H. Matsui, M. C. Lin, and Y. P. Lee, Nat. Chem. 6, 477 (2014). 6J. M. Beames, F. Liu, L. Lu, and M. I. Lester, J. Am. Chem. Soc. 134, 20045 (2012). 7M. Nakajima and Y. Endo, J. Chem. Phys. 139, 101103-1 (2013). 8O. Horie and G. K. Moortgat, Acc. Chem. Res. 31, 387 (1998). 9R. L. Mauldin III, T. Berndt, M. Sipilä, P. Paasonen, T. Petäjä, S. Kim, T. Kurtén, F. Stratmann, V. M. Kerminen, and M. Kulmala, Nature 488, 193 (2012). 10(a) J. M. Beames, F. Liu, L. Lu, and M. I. Lester, J. Chem. Phys. 138, 244307 (2013); (b) F. Liu, J. M. Beames, A. S. Petit, A. B. Mccoy, and M. I. Lester, Science 345, 1596 (2014); (c) F. Liu, J. M. Beames, A. M. Green, and M. I. Lester, J. Phys. Chem. A 118, 2298 (2014). 11C. A. Taatjes, O. Welz, A. J. Eskola, J. D. Savee, D. L. Osborn, E. P. F. Lee, J. M. Dyke, D. W. K. Mok, D. E. Shallcross, and C. J. Percival, Phys. Chem. Chem. Phys. 14, 10391 (2012). 12S. Hatakeyama, H. Kobayashi, Z. Y. Lin, H. Takagi, and H. Akimoto, J. Phys. Chem. 90, 4131 (1986). 13Z. J. Buras, R. M. I. Elsamra, A. Jalan, J. E. Middaugh, and W. H. Green, J. Phys. Chem. A 118, 1997 (2014). 14L. Vereecken, H. Harder, and A. Novelli, Phys. Chem. Chem. Phys. 16, 4039 (2014). 15B. Ouyang, M. W. McLeod, R. L. Jones, and W. J. Bloss, Phys. Chem. Chem. Phys. 15, 17070 (2013). 16T. Berndt, T. Jokinen, R. L. Mauldin, T. Petäjä, H. Herrmann, H. Junninen, P. Paasonen, D. R. Worsnop, and M. Sipilä, J. Phys. Chem. Lett. 3, 2892 (2012). 17A. B. Ryzhkov and P. A. Ariya, Phys. Chem. Chem. Phys. 6, 5042 (2004); P. Neeb, O. Horie, and G. K. Moortgat, J. Phys. Chem. A 102, 6778 (1998). 18(a) R. A. Alvarez and C. B. Moore, J. Phys. Chem. 98, 174 (1994); (b) W. Sander, Angew. Chem., Int. Ed. Engl. 24, 988 (1985); (c) J. M. Anglada, J. M. Bofill, S. Olivella, and A. Sole, J. Am. Chem. Soc. 118, 4636 (1996); (d) L. B. Harding and W. A. Goddard III, J. Am. Chem. Soc. 100, 7180 (1978); (e) R. Gutbrod, E. Kraka, R. N. Schindler, and D. J. Cremer,
J. Chem. Phys. 142, 124312 (2015) J. Am. Chem. Soc. 119, 7330 (1997); (f) J. H. Kroll, S. R. Sahay, J. G. Anderson, and K. L. Demerjian, J. Phys. Chem. A 105, 4446 (2001). 19(a) P. F. Lee, H. Matsui, W. Y. Chen, and N. S. Wang, J. Phys. Chem. A 116, 9245 (2012); (b) H. Su, W. Mao, and F. Kong, Chem. Phys. Lett. 322, 21 (2000); (c) J. M. Anglada, R. Crehuet, and J. M. Bofill, Chem. - Eur. J. 5, 1809 (1999). 20(a) M. T. Nguyen, T. L. Nguyen, V. T. Ngan, and H. M. T. Nguyen, Chem. Phys. Lett. 48, 183 (2007); (b) E. Miliords, K. Ruedenberg, and S. S. Xantheas, Angew. Chem., Int. Ed. 52, 5736 (2013); (c) J. Kalinowski, M. Rasanen, P. Heinonen, I. Kilpelainen, and R. B. Gerber, Angew. Chem., Int. Ed. 53, 265 (2014). 21D. Townsend, S. A. Lahankar, S. K. Lee, S. D. Chambreau, A. G. Suits, X. Zhang, J. Rheinecker, L. B. Harding, and J. M. Bowman, Science 306, 1158 (2004); P. L. Houston and S. H. Kable, Proc. Natl. Acad. Sci. U. S. A. 105, 16079 (2006); R. S. Zhu and M. C. Lin, Chem. Phys. Lett. 478, 11 (2009). 22M. J. Frisch et al., G 09, Revision D.01, Gaussian, Inc., Wallingford, CT, 2009. 23See supplementary material at http://dx.doi.org/10.1063/1.4914987 for full PES of CH2OO and CH3CHOO, optimized geometries of reactants, intermediates and transition states, IRC figure, vibrational frequencies, and predicted rate coefficients. 24K. A. Peterson, D. E. Woon, and T. H. Dunning, Jr., J. Chem. Phys. 100, 7410 (1994). 25V. Mokrushin, V. Bedanov, W. Tsang, M. Zachariah, and V. Knyazev, ChemRate, version 1.5.8, NIST, Gaithersburg, MD, 2009. 26J. H. Lehman, H. Li, J. M. Beames, and M. I. Lester, J. Chem. Phys. 139, 141103 (2013). 27(a) D. C. Fang and X. Y. Fu, J. Phys. Chem. A 106, 2988 (2002); (b) B.-Z. Chen, J. M. Anglada, M.-B. Huang, and F. Kong, J. Phys. Chem. A 106, 1877 (2002); (c) J. G. Chang, H. T. Chen, S. Xu, and M. C. Lin, J. Phys. Chem. A 111, 6789 (2007). 28C. K. Huang, Z. F. Xu, M. Nakajima, H. M. T. Nguyen, M. C. Lin, S. Tsuchiya, and Y.–P. Lee, J. Chem. Phys. 137, 164307 (2012). 29J. M. Anglada, P. Aplincourt, J. M. Bofill, and D. Cremer, ChemPhysChem 3, 215 (2002). 30K. T. Kuwata, M. R. Hermes, M. J. Carlson, and C. K. Zogg, J. Phys. Chem. A 114, 9192 (2010).
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THE JOURNAL OF CHEMICAL PHYSICS 142, 124312 (2015)
A novel and facile decay path of Criegee intermediates by intramolecular insertion reactions via roaming transition states Trong-Nghia Nguyen,1,2 Raghunath Putikam,1 and M. C. Lin1,a) 1
Department of Applied Chemistry and Institute of Molecular Science, National Chiao Tung University, Hsinchu 30010, Taiwan 2 Department of Physical Chemistry, Hanoi University of Science and Technology, Hanoi, Vietnam
(Received 2 December 2014; accepted 3 March 2015; published online 27 March 2015) We have discovered a new and highly competitive product channel in the unimolecular decay process for small Criegee intermediates, CH2OO and anti/syn-CH3C(H)OO, occurring by intramolecular insertion reactions via a roaming-like transition state (TS) based on quantum-chemical calculations. Our results show that in the decomposition of CH2OO and anti-CH3C(H)OO, the predominant paths directly produce cis-HC(O)OH and syn-CH3C(O)OH acids with >110 kcal/mol exothermicities via loose roaming-like insertion TSs involving the terminal O atom and the neighboring C–H bonds. For syn-CH3C(H)OO, the major decomposition channel occurs by abstraction of a H atom from the CH3 group by the terminal O atom producing CH2C(H)O–OH. At 298 K, the intramolecular insertion process in CH2OO was found to be 600 times faster than the commonly assumed ring-closing reaction. C 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4914987]
I. INTRODUCTION
Criegee intermediates (CIs) are key reaction intermediates in the reactions of ozone (O3) with olefins1 and the reaction of O2 with 3CH2, a key intermediate in the combustion of hydrocarbons.2 Small CIs have been recently detected experimentally,3–7 including the 2 conformers of acetaldehyde oxide, syn-CH3C(H)OO and anti-CH3C(H)OO, which differ in the orientations of the C–O–O group.4(b) Of the CIs formed during ozonolysis, ∼50% of them decompose to produce OH on a sub second timescale while the other 50% are stabilized.8–10 These CIs can then react with other components in the air or decompose over a much longer lifetime, t < 0.3 s, forming OH radicals. Bimolecular reactions between CIs and H2O, SO2, NO2, CH3C(O)H, CF3C(O)CF3, CH3C(O)CH3, HCHO, and so on have been reported in the literature.8,9,11–18 Experimental studies on the CH2OO reactions with SO2 and NO2 proved to be unexpectedly rapid, implying a substantially greater role of the carbonyl oxides in models of tropospheric sulfate and nitrate chemistry than previously assumed.4(a) The reactions of the small CIs, CH2OO and CH3CHOO, with carbonyl species were measured by laser photolysis/tunable synchrotron photoionization mass spectrometry.11 The results showed that the rate coefficient for CH2OO + hexafluoroacetone was k = (3.0 ± 0.3) × 10−11 cm3 molecule−1 s−1, supporting the possible use of hexafluoroacetone as a Criegee-intermediate scavenger. The mechanism for decomposition of CIs has been reported in the literature.1(c),17–19 Sander found evidence that the O(3P) atom was ejected in the reaction of a phenyl carbene with O2 by RR′COO → RR′CO + O(3P).18(b) However, for the singlet H2COO(1A′′), Anglada et al.18(c) using complete a)Author to whom correspondence should be addressed. Electronic mail:
[email protected]. 0021-9606/2015/142(12)/124312/6/$30.00
active space self-consistent field (CASSCF) and multireference single and double configuration interaction (MRDCI) calculations showed that it correlates with the singlet products H2CO(1A1) and O(1D), with a much higher energy than H2COO(1A′′). Therefore, the lower possible reaction paths of the singlet H2COO(1A′′) have been suggested,18(a),18(c)
For anti-CH3C(H)OO, a similar decomposition mechanism is generally believed to give OH + CH3CO, CH4 + CO2, CH2CO + H2O, and CH3OH + CO.1(c) For syn-monosubstituted CIs, such as syn-CH3C(H)OO, tautomerization to the corresponding hydroperoxide involving a 1,4 H-atom migration from the methyl to the terminal O appears to be a competitive isomerization pathway.1(c),18(c) However, in the above calculations for the small Criegee systems, uncertainties still exist regarding the tight transition states (TSs) forming dioxiranes from CH2OO and antiCH3CHOO. Recent multireference calculations show that Criegee intermediate, CH2OO, is predominantly zwitterionic with only weak biradical character.5,20 In addition, under a high concentration condition, the zwitterionic CH2OO was recently found to decay bimolecularly via a 6-member-ring dimeric intermediate by a head-to-tail association mechanism rapidly producing exothermic products, 2 CH2O + O2(1∆) (∆rH = −75.9 kcal/mol).5(b) Under a low concentration condition, in addition to the dioxirane cyclization path, is it also possible for the very exothermic intramolecular reaction directly transforming CH2OO to HCOOH? Such a unimolecular decay process is not unlikely on grounds that the terminal O atom should at least partially possess the O(1 D) character
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when the O–O bond is stretched; if so, it may undergo insertion into a cis-C–H bond forming HC(O)OH and CH3C(O)OH with >110 kcal/mole exothermicities which may enthalpically drive the reactions into the deep wells of the acids. HCOOH and CH2OO are known products of the Criegee reaction involving O3 and C2H4. There is no experimental measurement for the formation of formic acid from the unimolecular reaction CH2OO under clean molecular beam conditions however. Welz et al.4(a) have reported the direct photoionization mass spectrometric detection of formaldehyde oxide (CH2OO) as a product of the reaction of CH2I with O2. Their photoionization spectrum of the m/z = 46 species agrees with that experimentally measured system of CH2OO in chlorine-initiated dimethyl sulfoxide (DMSO) oxidation3 and with coupledcluster with single and double and perturbative triple excitations/complete basis set (CBS) extrapolation calculations for the CH2OO ionization energy.20(a) Dioxirane and formic acid have much higher ionization energies, 10.82 eV and 11.33 eV,16,17 respectively. The mass and the photoionization spectra have well supported the product as formaldehyde oxide, CH2OO. At longer times, a small formic acid signal was observed which may be produced by the isomerization of CH2OO. In this work, we have carried out a detailed mapping of the potential energy surfaces (PESs) of the two small CIs with a particular emphasis on the search for the existence of the intramolecular insertion mechanism via a loose transition state in which the terminal O atom exhibits its O(1D) character. Such a novel reaction path was indeed found for the first time via a roaming-like transition state21 and shown to be a facile decay path in both systems as well. II. COMPUTATIONAL METHODS
We have characterized the mechanisms of these two reactions by quantum-chemical calculations using hybrid density functional theory (B3LYP) and MP2 methods with the aug-ccpVTZ basis set to optimize geometries of all species involved by means of the Gaussian 09 package.22 Further improvement of energetics of the PESs has been made by singlepoint calculations with CCSD(T)/aug-cc-pVTZ. The predicted full PESs of CH2OO and CH3CHOO are presented in Figures S1 and S2 of the supplementary material23 and the corresponding lower energy paths are shown in Figures 1 and 2, respectively. The transition state geometries are used as an input for intrinsic reaction coordinate (IRC) calculations to verify the connectivity of the reactants and products for further confirmation. For the new roaming-like loose transition state of the CH2OO system, it has been confirmed with optimizations using several different methods including UB3LYP, UBLYP, UCCSD, and CASSCF(14,12) employing the aug-cc-pVTZ basis set. To more accurately evaluate the energies, we employed the UCCSD(T)/CBS method24 based on the CCSD/augcc-pVTZ optimized geometries for the two critical transition states, TS1 and TS2, of CH2OO to be described in detail below. The CBS energies were evaluated with these geometries as follows. The total energies E(X) computed with the aug-ccpVXZ basis sets (X = 2, 3, 4) extrapolated to the CBS limits ECBS employing a three-point extrapolation scheme,24 E (X) = ECBS + b exp [− (X − 1)] + c exp[−(X − 1)2],
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FIG. 1. Energy diagrams for the CH2OO decomposition. All values (in kcal/mol at 0 K) were computed at the CCSD(T)/aug-cc-pVTZ//B3LYP/augcc-pVTZ level. The values in parentheses are calculated by using the UCCSD(T)/CBS//UCCSD/aug-cc-pVTZ level. Data in the square bracket are predicted at CCSD(T)/aug-cc-pVTZ//CAS(8,8)/cc-pVTZ level.
where E(X) is the single point energy calculated by UCCSD (T)/aug-cc-pVTXZ method, X is the cardinal number of the basis sets connected with X = 2 (DZ), 3 (TZ), 4 (QZ), and ECBS, b, and c are parameters to be fitted. Calculations of the micro canonical rate coefficients for the low-lying reaction channels were made with the Chemrate program25 based on the Rice–Ramsperger–Kassel–Marcus (RRKM) theory with Eckart tunneling corrections. Chemical activation, isomerization, and decomposition were accounted for with
FIG. 2. The IRC profile for the loose TS1, computed at the UB3LYP/aug-ccpVTZ level.
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the program through the solution of the time-dependent onedimensional master equation. In the calculation of the specific rate constant, the number of available states in the transition state is obtained at the energy E and with the total angular momentum J resolved based on the rigid-rotor harmonic oscillator (RRHO) assumption for the energy levels. Our predicted rate constants were calculated from conventional (fixed geometry) canonical transition-state theory. More detailed information is available in the supplementary material.23
III. RESULTS AND DISCUSSION A. Potential energy surface of CH2OO decomposition
Aside from the commonly invoked ring cyclization reaction producing cyc-H2COO (IS3), we have investigated three other possibilities: H-atom migration to the neighboring and terminal O atoms producing HCO(H)O and HCO–OH, respectively, as well as the intramolecular O-atom insertion into a C–H bond as alluded to above, when the terminal O-atom is stretched along the O–O dissociation path producing CH2O + O(1D), whose dissociation limit is now known to require a rather high energy (∼54 kcal/mol)26 and thus is not accessible thermochemically under ambient atmospheric conditions. The optimized geometries of CH2OO bond lengths are in good agreement with the experimental and theoretical data as shown in Table S1.5,7,23 The vibrational frequencies of all the species involved in these reactions are predicted at the B3LYP/aug-cc-pVTZ level and are given in Table S2.23 The new roaming-like mechanism will be discussed below after the presentation for the ring cyclization and H-transfer processes. The formation of the cyclic cyc-H2COO (IS3: −25.7 kcal/mol) occurs via transition state TS2 (18.7 kcal/mol); the IRC result shows that TS2 connects IS3 with CH2OO. These results are in good agreement with those in the literature1(c),27(a),27(b) (Table S3 and Figure S323). The relative energies of IS3 and TS2 predicted by CCSD(T)/aug-ccpVTZ//B3LYP/aug-cc-pVTZ, −25.7 and 18.7 kcal/mol, and by CCSD(T)/aug-cc-pVTZ//MP2/aug-cc-pVTZ, −26.5 and 18.7 kcal/mol, are close to −25.1 and 19.6 kcal/mol, respectively, by Fang et al.27(a) at the CAS(8,6)+1+2/cc-pVDZ level of theory (Table S323). Because of the low barrier, 18.7 kcal/mol, this channel has been commonly assumed to be predominant and rate-controlling in the fragmentation of CH2OO. The next two channels involve H-migration forming HCO–OH (IS8: 7.1 kcal/mol) via TS16 lying 30.4 kcal/mol above the reactant and HCO(H)O (IS9: 70.5 kcal/mol) via TS17 lying 92.6 kcal/mol above the reactant. Clearly, these processes are insignificant because of their high energy barriers and endothermicities (Figure S1).23 On the other hand, the direct transformation of CH2OO into c–HC(O)OH (IS1) is very exothermic (−111.3 kcal/mol); as alluded to above, such a transformation process was indeed found after an exhaustive manual search. The very loose transition state TS1 was found to lie 16.1 kcal/mol above CH2OO or 2.6 kcal/mol below the commonly known TS2 (18.7 kcal/mol) for cyclization; the barrier difference is similar to the UCCSD(T)/CBS//UCCSD/aug-cc-pVTZ
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value, 2.2 kcal/mol (Figure 1). TS1 has a unique imaginary frequency of 273.7i corresponding to the insertion of the terminal O atom into a C–H bond. The H–O and O–O bond distances, 1.997 and 2.639 Å, are close to the structure of the transition state corresponding to those for the insertion of the O atom.28 The IRC profile of TS1 is displayed in Figure 2. The result clearly shows that TS1 does in fact smoothly connect CH2OO and c-HC(O)OH. The existence of this roaming-like TS, as mentioned above, has been confirmed with optimizations using several computational methods: UB3LYP, UBLYP, UCCSD, and CASSCF(14,12) employing the aug-cc-pVTZ basis set. The optimized geometries with all the methods are shown in Figure S4.23 The corresponding barrier energies and vibrational frequencies estimated at different levels of the theory are summarized in Tables S4 and S5,23 respectively. As revealed by the PES, there are two possible reaction paths for cycH2COO (dioxymethane) decomposition: H2 elimination and ring opening reactions. First, the intermediate dioxymethane can dissociate by H2 elimination through TS6 to yield H2 + CO2 with a barrier of 24.9 kcal/mol. In the second mechanism by ring opening via TS6a, we were unable to locate the TS and open-H2C(O)O with the B3LYP method; it was, however, located with a higher level of theory by CCSD(T)/aug-cc-pVTZ//CAS(8,8)/cc-pVTZ. As shown in the PES, cyc-H2COO can decompose into open-H2C(O)O by O–O bond breaking via TS6a with a 21.1 kcal/mol barrier, to be followed by a rapid H-atom migration to the neighboring O atom to produce the trans-formic acid (t-HC(O)OH) via TS6b with a small 2.4 kcal/mol barrier. The optimized geometries of these processes are shown in Figure S5.23 Anglada and coworkers19(c),27(b) have studied the detailed decomposition mechanism of cyc-H2COO at the CASPT2/6-31g(d,p)//CASSCF/6-31g(d,p) level of theory. Their barriers for the ring opening and H-migration were reported to be 18.5 and 1.9 kcal/mol, respectively.27(b) Fang et al.27(a) also have calculated the similar barrier energies at the CAS(8,6)+1+2/cc-pVDZ level to be 19.9 and 1.6 kcal/mol. These results are consistent with our values cited above. For CH2OO decomposition, TS1 and TS2 are the rate-controlling steps; the details of other remaining dissociation processes are shown in Figures 1 and S1.23 The thermal decomposition of 1CH2OO producing 1H2CO + O(1D) and 3 H2CO + O(3P) products predicted at the CCSD(T)/aug-ccpVTZ//B3LYP/aug-cc-pVTZ level are endothermic by 54.2 and 74.2 kcal/mol, respectively. Clearly, the 3H2CO + O(3P) formation is not favored energetically. In addition, we have constructed the approximate minimum energy path (MEP) for 3 H2CO + O(3P) formation by freezing the 3H2CO structure when it interacts with the O(3P) atom as shown in Fig. S6,23 computed at the CASPT2/aug-cc-pVTZ//CAS(8,8) augcc-pVTZ level. Significantly, at any point on the triplet product curve, a full optimization immediately converges the structure and energy onto the singlet MEP curve. As shown in Fig. S6,23 a full optimization at 3.0 Å, when O atom approaches near the bend 3CH2O, it immediately converges to the 1CH2O–O(1D) curve with the release of over 70 kcal/mol π bond energy. Thus, the unimolecular decomposition of the singlet CH2OO is not favored to produce the 3H2CO + O(3P)
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products. In addition, detailed kinetics and mechanism for the decomposition of HCOOH have been reported previously by Chang et al.27(c) Based on the mechanism, the highly excited c-HC(O)OH thus formed can decompose barrierlessly giving HCO + OH (P3: −9.9 kcal/mol), H2 + CO2 (P1: −119.6 kcal/mol) via TS5 (−45.6 kcal/mol), or t-HC(O)OH (IS5: −115.3 kcal/mol) via a low isomerization barrier TS3 (−104.0 kcal/mol). Similarly, t-HC(O)OH can decompose barrierlessly forming HCO + OH (P3: −9.9 kcal/mol) or H2O + CO (P2: −111.3 kcal/mol) via TS4 (−48.8 kcal/mol). Our predicted energies are in good agreement with previous studies27 (Table S323). The direct CH2OO → HC(O)OH channel via the novel roaming-like TS1 not only has a very loose transition structure but also has the lowest energy barrier, 16.1 kcal/mol. The main products of the reaction are expected to be OH, H, CO, and CO2 from the fragmentation of the highly excited HC(O)OH as observed experimentally.1(c),9,16,27,29 B. Potential energy surface of CH3C(H)OO decomposition
The detailed PES of the CH3CHOO system is presented in Figure S2 in the supplementary material,23 only the abbreviated one depicting the lowest energy paths is shown in Figure 3. The related geometries are shown in Figure S7.23 Corresponding vibrational frequencies of all species calculations at the B3LYP/aug-cc-pVTZ level are summarized in Table S6.23 There are two conformers of CH3C(H)OO, the anti-structure (a-CH3C(H)OO) and the synstructure (s-CH3C(H)OO), with the former having a higher energy, 3.5 kcal/mol. The predicted energy barrier for the isomerization, anti → syn, is as high as 38.1 kcal/mol. Therefore, it should have different reactivities for these conformers. These results are in good agreement with
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experimental studies,4(b),10,11 in which the two conformers were independently detected, and the fit parameters suggested a far larger overall production (90%) of the more stable synconformer, assuming that the electronic transition moments for the two conformers were similar. Our result reveals that the decomposition of aCH3C(H)OO has two lowest energy channels, 15.4 and 15.6 kcal/mol, respectively, to the cyclization giving cycCH3C(H)OO (IS6: −26.3 kcal/mol) via TS8 and the intramolecular insertion forming s-CH3C(O)OH (IS4: −115.3 kcal/mol) via TS7, which has a unique imaginary frequency of 126.3i with the H–O and O–O bond distances, 1.956 and 2.573 Å. The IRC analysis also confirms the transition state TS7 (Figure S8).23 The predicted relative energies of TS8 and the cyc-CH3C(H)OO at the CCSD(T)/augcc-pVTZ//B3LYP/aug-cc-pVTZ level, 15.4 kcal/mol and −26.3 kcal/mol, agree closely with the values, 15.4 kcal/mol and −23.44 kcal/mol, 17.55 kcal/mol and −28.2 kcal/mol, using MCG3//QCISD/MG3 and BB1K/6-31+G(d,p), respectively.30 Although the two channels have similar energy barriers, the intramolecular insertion via TS7 is expected to be dominant because of its very loose roaming-like TSstructure as illustrated below with the predicted rate constants. The major products of the reaction should, therefore, derive primarily from the decomposition of a/s-CH3C(O)OH, such as OH, H, and CO2, consistent with experimental results.1(c),9,10,16,20,27–30 The decomposition of the s-CH3C(H)OO has the lowest energy channel, 16.6 kcal/mol, corresponding to the abstraction of a H atom in CH3 by the terminal O atom forming CH2C(H)O–OH (IS7: −22.2 kcal/mol) via TS12. Our barrier energy is in good agreement with the experimental value of 16.0 kcal/mol reported by Liu et al.10(b) Recent theoretical results give ∼17.9 kcal/mol.10(c),30 However, the attacking of the terminal O atom at the C atom forming cyc-CH3C(H)OO (IS6) via TS13 has to overcome 23.3 kcal/mol barrier. These results are in close agreement with previous theoretical studies (Table S7).23 C. Rate constant calculations
We have predicted the microcanonical rate coefficients and the thermal rate constants for the unimolecular reactions of CH2OO and anti/syn-CH3C(H)OO producing various products via insertion and ring formation paths based on the TS’s presented above computed at the CCSD(T)/aug-ccpVTZ//B3LYP/aug-cc-pVTZ level as follows: CH2OO → c-HC (O) OH (k 1) , CH2OO → cyc-H2C (O) O (k2) , a-CH3C (H) OO → CH3C (O) OH (IS4) (k3) , a-CH3C (H) OO → c-CH3C (H) OO (k 4) , s-CH3C (H) OO → CH2C (H) OO(IS7) (k5) , s-CH3C (H) OO → c-CH3C (H) OO(IS6) (k6) . FIG. 3. Energy diagrams for the anti/syn-CH3C(H)OO decomposition. All values (in kcal/mol at 0 K) were computed at the CCSD(T)/aug-ccpVTZ//B3LYP/aug-cc-pVTZ level.
The vibrational frequencies of all the species involved in these reactions are calculated at the B3LYP/aug-cc-pVTZ level and are listed in Tables S2 and S6 for the kinetic
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J. Chem. Phys. 142, 124312 (2015)
298 K values.23 The microcanonical rate coefficients for both CH2OO and CH3C(H)OO decomposition channels are shown in Figure 4 to illustrate their relative importance. In Figure 4(a), k 1(E) for generation of c-HC(O)OH via the insertion process is greater than k2(E) for the formation of cyc-H2C(O)O via ring formation by 2-3 orders of magnitude over a wide range of energies and the integrated thermal rate constant at 298 K for the former is 600 times greater. The rate coefficients for the anti-CH3C(H)OO decomposition as shown in Figure 4(b), k3 (E) for CH3C(O)OH(IS4) via insertion into the CH group is also dominant over that for the ring formation producing cyc-CH3C(H)OO. In the case of syn-CH3C(H)OO shown in Figure 4(c), H migration giving CH2C(H)OOH (IS7) (k5) is dominant. It should be mentioned that our effort to look for the existence of a transition state for insertion of the terminal O atom into a C–H bond of the methyl group failed to locate it after an extensive search.
IV. CONCLUSION
To summarize, the PESs and decomposition mechanisms of CH2OO and syn/anti-CH3C(H)OO have been computed at the CCSD(T)/aug-cc-pVTZ//B3LYP/aug-cc-pVTZ and CCSD(T)/aug-cc-pVTZ//MP2/Aug-cc-pVTZ levels; the energy dependent decomposition rate constants have been predicted. The results show that the isomerization of CH2OO forming cis-HC(O)OH via a very loose roaming-like insertion TS lying 16.1 kcal/mol above CH2OO is very facile and could be competitive with the fastest known channels. CH3C(H)OO has two structures, anti- and syn-, in which the syn-structure is more stable, and the inter-conversion (anti → syn) energy barrier is rather high, 38.1 kcal/mol. The major channel in the decomposition of anti-CH3C(H)OO is the insertion of the terminal O atom into the neighboring C–H bond forming syn-CH3C(O)OH, whereas the dominant channel in the syn-CH3C(H)OO decomposition reaction is the abstraction of a H atom from the CH3 group by the terminal O atom producing CH2C(H)O–OH. In both systems, the commonly assumed cyclization process producing dioxiranes with their subsequent isomerization/decomposition reactions is much less competitive. ACKNOWLEDGMENTS
FIG. 4. Predicted rate coefficients of the unimolecular decomposition channels of (a) CH2OO, (b) anti-CH3CHOO, and (c) syn-CH3CHOO, as a function of energy calculated by using CCSD(T)/aug-cc-pVTZ//B3LYP/aug-cc-pVTZ method.
calculations.23 The high-pressure and low-pressure limit rate constants for the dissociation processes predicted for the 200-1000 K temperature range obtained by least-squares fitting have been presented in Table S8 along with the
The Ministry of Science and Technology of Taiwan (Grant No. NSC102-2745-M009-001-ASP) and the Ministry of Education, Taiwan (“Aim for the Top University Plan” of National Chiao Tung University) supported this work. The National Center for High-performance Computing provided computer time. 1(a)
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